During WWI, it was discovered that you can add a chemical called tetraethyl lead to gasoline and significantly improve its octane rating. Cheaper grades of gasoline could be made usable by adding this chemical. This led to the widespread use of "ethyl" or "leaded" gasoline. Unfortunately, the side effects of adding lead to gasoline are:

Lead clogs a catalytic converter and renders it inoperable within minutes.

The Earth became covered in a thin layer of lead, and lead is toxic to many living things (including humans).

When lead was banned, gasoline got more expensive because refineries could not boost the octane ratings of cheaper grades any more. Airplanes are still allowed to use leaded gasoline, and octane ratings of 115 are commonly used in super-high-performance piston airplane engines (jet engines burn kerosene, by the way).

Another common additive is MTBE. MTBE is the acronym for methyl tertiary butyl ether, a fairly simple molecule that is created from methanol. .

MTBE gets added to gasoline for two reasons:

It boosts octane

It is an oxygenate, meaning that it adds oxygen to the reaction when it burn; Ideally, an oxygenate reduces the amount of unburned hydrocarbons and carbon monoxide in the exhaust.

MTBE started getting added to gasoline in a big way after the Clean Air Act of 1990 went into effect. Gasoline can contain as much as 10 percent to 15 percent MTBE.

The main problem with MTBE is that it is thought to be carcinogenic and it mixes easily with water. If gasoline containing MTBE leaks from an underground tank at a gas station, it can get into groundwater and contaminate wells. Of course, MTBE isn't the only thing getting into the groundwater when a tank leaks -- so is gasoline and a host of other gasoline additives.

According to this page at the EPA:

Although there is no established drinking-water regulation, USEPA has issued a drinking-water advisory of 20 to 40 micrograms per liter (µg/L) on the basis of taste and odor thresholds. This advisory concentration is intended to provide a large margin of safety for noncancer effects and is in the range of margins typically provided for potential carcinogenic effects.

The most likely thing to replace MTBE in gasoline is ethanol -- normal alcohol. It is somewhat more expensive than MTBE, but it is not a cancer threat.

Hydrocracking – a severe, high temperature, high pressure refinery process that converts heavy black oil into gasoline and diesel fuel by cracking or breaking up larger molecules, in the presence of hydrogen and a catalyst.

Hydrotreating – a refinery process that reacts a fraction of crude oil with hydrogen at high temperature and pressure, in the presence of a catalyst, to improve colour and odour, and reduce sulphur content.